This invention relates to thermal spray powders of dicalcium silicate, thermal spray coatings thereof, and a process for manufacturing such powders.
Thermal spraying involves the melting or at least heat softening of a heat fusible material such as a metal or ceramic, and it propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. In a plasma type of thermal spray gun, a high temperature stream of plasma gas heated by an arc is used to melt and propel powder particles. Other types of thermal spray guns include a combustion spray gun in which powder is entrained and heated in a combustion flame, such as a high velocity, oxygen-fuel (HVOF) gun. Thermal spray coatings of oxide ceramics are well distinguished from other forms such as sintered or melt casted by a characteristic microstructure of flattened spray particles visible in metallographically prepared cross sections of coatings.
In one group of thermal spray materials, powders are formed of oxides for spraying coatings that are used for thermal insulation at high temperature such as on burner can surfaces in gas turbine engines. Coatings are also needed for erosion and wear protection at high temperatures, and require resistance against thermal cycle fatigue and hot corrosion in a combustion environment. Zirconium dioxide (zirconia) typically is used in such applications. Because of phase transitions, the zirconia is partially or fully stabilized with about 5% (by weight) 15% calcium oxide (calcia) or 6% to 20% yttrium oxide (yttria).
However, these coatings have limitations particularly in resistance to hot corrosion as they allow attack of the substrate or a bond coating
Dicalcium silicate (Ca2SiO4) is a ceramic conventionally used for cement and refractory applications. Excellent hot corrosion and heat resistance of dicalcium silicate based coatings also has been demonstrated in a high temperature combustion environment. However, it is polymorphic with at least five phases including three high temperature xcex1 modifications, an intermediate temperature monoclinic xcex2 phase (larnite) and an ambient temperature xcex3 phase. The transformation from the xcex2 phase to the xcex3 phase exhibits a volume increase of 12% leading to degradation in both the thermal spray process and the coatings in thermal cycling. The xcex2 phase may be retained by quenching or the use of a stabilizer such as sodium or phosphorous. Other suggested stabilizers include oxides (or ions) of sulphur, boron, chromium, arsenic, vanadium, manganese, aluminum, iron, strontium, barium and potassium. At least some of these have also been reported as unsuccessful, and therefore still questionable in stabilizing, including chromium, aluminum, iron, strontium and barium.
U.S. Pat. No. 4,255,495 (Levine et al.) discloses plasma sprayed coatings of thermal barrier oxides containing at least one alkaline earth silicate such as calcium silicate. U.S. Pat. No. 5,082,741 (Tiara et al.) and an article xe2x80x9cAdvanced Thermal Barrier Coatings Involving Efficient Vertical Micro-Cracksxe2x80x9d by N.Nakahira, Y.Harada, N.Mifune, T.Yogoro and H.Yamane, Proceedings of International Thermal Spray Conference, Orlando FL., May 28-Jun. 5, 1992, disclose thermal spray coatings of dicalcium silicate combined with calcium zirconate (CazrO3) in a range of proportions.
A commercial powder of xcex2 phase dicalcium silicate for thermal spraying is sold by Montreal Carbide Co. Ltd., Boucherville CQ, Canada, indicated in their xe2x80x9cTechnical Bulletin MC-C2Sxe2x80x9d (undated).
In a chemical analysis the present inventors measured less than 1% by weight of potential stabilizers such as phosphorous in Montreal Carbide powder.
A commercial powder of dicalcium silicate for thermal spraying also is sold by Cerac Inc., Milwaukee, Wisconsin. In a Certificate of Analysis for calcium silicate (Oct. 20, 1997), Cerac reports major xcex2 phase and low levels of aluminum (0.12%), iron (0.1%) and magnesium (0.25%), and 0.02% or less of other elements.
An object of the present invention is to provide an improved powder of dicalcium silicate for thermal sprayed coatings for thermal barriers having resistance to hot corrosion and sulfidation in a combustion environment. A further object is to provide a novel process of manufacturing such a powder. Another object is to provide an improved thermal sprayed coating of dicalcium silicate for thermal barriers having resistance to hot corrosion and sulfidation in a combustion environment.
The foregoing and other objects are achieved by a thermal spray powder comprising a substantially uniform powder composition consisting of dicalcium silicate, sodium, a further ingredient selected from the group consisting of phosphorous and zirconium, and incidental ingredients, such that the dicalcium silicate is stabilized in a larnite phase that is majority by volume. In one embodiment the further ingredient comprises phosphorous, in which case, preferably, the sodium recited as disodium monoxide is present in an amount of about 0.2% to 0.8%, and the phosphorous recited as phosphorous pentoxide is present in an amount of about 2.5% to 4%. In another embodiment the further ingredient comprises zirconium, in which case, preferably, the sodium recited as disodium monoxide is present in an amount of about 0.2% to 0.8%, and the zirconium recited as zirconium dioxide is present in an amount of about 10% to 50%. These percentages are by weight of oxide based on the total composition. The zirconium, if present, should be at least partially in the form of zirconium dioxide containing calcium oxide as stabilizer of the zirconium dioxide, or yttrium oxide its stabilizer.
Objectives also are achieved by a process of manufacturing a thermal spray powder of dicalcium silicate having a stabilized crystal structure. An aqueous mixture is formed of calcium carbonate powder, silicon dioxide powder, and an organic binder containing as an-integral constituent a stabilizing element in an amount sufficient to stabilize the dicalcium silicate in a larnite phase that is majority by volume. The aqueous mixture is spray dried to form a powder. The spray dried powder is heated, such as by sintering or plasma melting, such that the dicalcium silicate is formed with larnite phase that is majority by volume.
Preferably the stabilizing element is sodium, advantageously contained in an organic binder sodium carboxymethylcellulose. Further advantageously, the aqueous mixture further comprises a compound of phosphorous, preferably as hydrous aluminum phosphate in aqueous solution. Alternatively or in addition to phosphorous, the aqueous mixture further comprises stabilized zirconium dioxide powder with calcia or yttria stabilizer.
Objectives are further achieved by a thermal spray coating of a composition as described above for the powder. The coating has a web of interconnected, randomly oriented microcracks substantially perpendicular to the coating surface. The coating may include a bonding layer of a thermal sprayed nickel or cobalt alloy on a metallic substrate, and an intermediate layer of a thermal sprayed partially or fully stabilized zirconium oxide. The layer of dicalcium silicate composition is thermal sprayed onto the intermediate layer. The intermediate layer blocks reaction between the bonding layer and the layer of dicalcium silicate composition.